Gravity and Magnetic Exploration- Principles, Practices, and Applications. Willam J. Hinze, Ralph R. B. Von Frese, Afif H. Saad. 2008

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Gravity and Magnetic Exploration Principles, Practices, and Applications

This combination textbook and reference manual provides a comprehensive account of the principles, practices, and application of gravity and magnetic methods for exploring the subsurface using surface, marine, airborne, and satellite measurements. Key current topics and techniques are described, including high-resolution magnetic investigations, time-variation gravity analysis from surface and satellite gravity measurements, absolute and gradient gravimetry, and the role of GPS in mapping gravity and magnetic ﬁelds. The book also describes the physical properties of rocks and other Earth materials that are critical to the effective design, implementation, and interpretation of surveys, and presents a thorough overview of digital data analysis methods used to process and interpret anomalies for subsurface information.

Each chapter starts with a general overview and concludes with a list of key concepts that help readers review what they have learned. An appendix provides a grounding on basic data analysis using simple and accessible mathematical notation.

End-of-chapter problem sets on an accompanying website, together with interactive computer-based exercises available online, give readers hands-on experience of processing, modeling, and interpreting gravity and magnetic anomaly data.

A comprehensive suite of full-color case histories on the book’s website illustrates the practical utility of modern gravity and magnetic surveys in energy, mineral, environmental, archaeological, and engineering exploration and lithospheric studies, as well as their potential limitations.

This book is an ideal text for advanced undergraduate and graduate courses, but also serves as a reference for research academics, professional geophysicists, and managers of exploration programs that include gravity and magnetic methods. It is a valuable resource for all those interested in petroleum, engineering, mineral, environmental, geological, and archeological exploration of the lithosphere.

WI L L I A M J . HI N Z Eis Emeritus Professor of Geophysics at Purdue University, and has taught exploration geophysics for 40 years. His extensive teaching and research experience is complemented by industrial consulting experience in the geological and engineering applications of gravity and magnetic ﬁelds. He has authored or co-authored more than 130 journal

publications, and four books, and has served as Associate Editor ofGeophysicsand Senior Editor ofThe Journal of Geophysical Research – Solid Earth. Professor Hinze has been a member of numerous government and scientiﬁc panels dealing with key issues from the geophysics and geology of continents to nuclear waste disposal and geophysical data. He is a member of the Society of Exploration Geophysicists, the American Geophysical Union, and the Geological Society of America.

RA L P H R . B . V O N FR E S Eis Professor of Earth Sciences at The Ohio State University, where he has taught undergraduate and graduate courses across geophysics and Earth systems since 1982. His research has focused mostly on archaeological and planetary applications of gravity and magnetic fields, and he has authored or co-authored more than 100 journal publications and served on several government and scientific panels. Professor von Frese is the founding co-chair of the Antarctic Digital Magnetic Anomaly Project (ADMAP), an international collaboration of the Scientific Committee for Antarctic Research (SCAR) and the International Association of Geomagnetism and Aeronomy (IAGA). He is a member of the Society of

Exploration Geophysicists, the American Geophysical Union, the Geological Society of America, and the International Association for Mathematical Geology.

AF I F H . SA A Dis a Geophysical Consultant to the oil and gas exploration industry, specializing in integrated interpretation, modeling, magnetic depth estimation, software development, and training. He has over 40 years of broad experience in the theory and practice of gravity and magnetic methods in exploration, including 25 years in the oil industry, and has provided training at several schools and workshops in basic, intermediate, and advanced gravity and magnetics in the United States and overseas. Dr Saad has authored or co-authored several publications, has peer-reviewed several papers and textbooks in gravity and magnetics forGeophysicsand theGeophysical Journal International, and has served as Associate Editor for Magnetic Exploration Methods for Geophysics. He is an active Emeritus member of the Society of Exploration Geophysicists (SEG), member of the SEG Gravity and Magnetics Committee, the SEG Global Affairs Committee, and the Geophysical Society of Houston.

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Gravity and Magnetic

Exploration

Principles, Practices, and Applications

William J. Hinze

Purdue University

Ralph R. B. von Frese

The Ohio State University

Aﬁf H. Saad

Saad GeoConsulting

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c a m b r i d g e u n i v e r s i t y p r e s s

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, S˜ao Paulo, Delhi, Mexico City

Cambridge University Press

The Edinburgh Building, CambridgeCB2 8RU, UK

Published in the United States of America by Cambridge University Press, New York www.cambridge.org

Information on this title: www.cambridge.org/9780521871013 CWilliam J. Hinze, Ralph R. B. von Frese and Aﬁf H. Saad 2012 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press.

First published 2012

Printed in the United Kingdom at the University Press, Cambridge A catalog record for this publication is available from the British Library Library of Congress Cataloging in Publication data

ISBN 978-0-521-87101-3 Hardback

Additional resources for this publication at www.cambridge.org/gravmag Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

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Preface

Objectives of this book

Investigations of terrestrial gravity and magnetic fields are among the oldest methods for determining the nature and processes of the Earth. Despite the development of an increasing number of additional complementary inves- tigative methods, some of which have better subsurface resolution, the gravity and magnetic methods continue to have an important, often decisive role, in a wide variety of terrestrial investigations. In contrast to the several available texts on specific topics in gravity and magnetics, this book provides an overall, modern resource on the principles, practices, and applications of both the gravity and magnetic methods to exploring the Earth. Although these aspects of the gravity and magnetic methods are well grounded in widely described and accepted principles, they are continually undergoing practical improvements and expanded understanding. The continued improvements result from enhanced technology for acquiring, processing, and interpreting data made possible largely by increasing computational power and new techniques. Spe- cial emphasis in gravity and magnetic exploration is being placed on high sensitivity mapping of anomalies at the extremities of their spectra, both shorter and longer wave- lengths, increasing the vertical and horizontal resolution of individual anomaly sources, and investigating the temporal variations in the gravity and magnetic fields of the Earth which permit new insights into Earth processes. It is hoped that this book which sets a benchmark in our current knowledge of gravity and magnetic exploration will encourage further developments in these methods and applications.

Both the theory and practice of gravity and magnetic methods applied to subsurface investigations are described in this book. The book considers the methods from the planning and organization of surveys through the analysis and interpretation of observations by digital computations incorporating both physical and geological princi-

ples. Case histories in supplemental chapters, which are available on an accompanying website illustrate the advan- tages and limitations of the methods in a variety of applications including near-surface engineering and archaeological studies, mineral and energy resource investigations, and planetary crustal and subcrustal studies. A ﬁnal chapter of this book summarizes these applications.

This presentation of gravity and magnetic methods differs substantially from existing books. For example, it takes into account the rapidly accelerating availability of subsurface, terrestrial, marine, airborne, and satellite measurements that the current information age is mapping in prodigious volumes. Further, it develops strategies on the combined use of these anomaly fields for solving subsurface problems. It also describes the newest stan- dards and methods for reducing gravity and magnetic data to a usable form, as well as modern instrumentation for observing both absolute and relative total field and vec- torial components. The book emphasizes practical procedures and applications, and the physical properties and geological principles that constrain subsurface analyses of gravity and magnetic fields. It describes all major modern topics of gravity and magnetic methods that are likely to be of general interest. More general descriptive material and procedures are highlighted, and reference is made to alternate procedures or approaches. However, necessarily numerous computational and interpretational procedures described in the geophysical literature which may have a more limited application are not included because of space limitations. Wherever possible, examples of these methods are briefly described and pertinent literature is cited to guide the interested reader to additional information.

Derivations of equations are restricted to those of general interest and details of many procedures are not included in the descriptions. Therefore the reader of this text will need to make abundant use of the many references cited for more detailed and comprehensive treatment of specialized topics.

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x Preface

The book can serve university courses in exploration and general Earth geophysics, gravity exploration, and magnetic exploration at the advanced undergraduate and beginning graduate levels. It uses mathematics up to and including basic differential equations, but develops the methods in simple digital array operations with minimal use of arcane and complex notations from integral and differential calculus. Thus, this book is much more compu- tationally oriented than most previous works because the reader can readily implement and explore analytical results with electronic computing. Consistent abbreviations and terminology are used throughout the book.

The book is not only a textbook, but also serves as a reference for practioners, that is for professional geophysicists, geologists, engineers, and other scientists who have limited experience in the application of the gravity and magnetic methods to subsurface problems. Especially useful in this regard are the supplemental case histories, which have extensive reference lists. The explanation of the gravity and magnetic methods along with the case histories will also facilitate the efforts of project managers to develop optimal strategies for implementing geophysical methods in solving subsurface problems. Thus, the book serves a wide audience ranging from advanced undergraduate and beginning graduate students in the Earth and planetary sciences and engineering to professional geoscientists and engineers in academia, government, and industry.

Related books

This comprehensive book is unique in the breath of the content and approach. However, a number of books per- taining to the gravity and magnetic exploration methods and their interpretation are available that the reader may ﬁnd to be useful supplements to this book. A selected list of these books includes Introduction to Potential Theory (Sigl, 1985), Interpretation of Filtered Grav- ity Maps(Steiner andZilahi-Sebess, 1988),Geo- physical Data Analysis (Menke, 1989), Geophysical Inverse Theory (Parker, 1994), Potential Theory in Gravity and Magnetic Applications(Blakely, 1995), Geologic Applications of Gravity and Magnetics: Case Histories(GibsonandMillegan, 1998),Geophysical Inverse Theory and Regularization Problems(Zhdanov, 2004),Principles of the Gravitational Method(Kauf- man and Hansen, 2008),Principles of the Magnetic Methods in Geophysics(Kaufmanet al., 2008),Gravity and Magnetic Interpretation in Exploration Geophysics (Murthy, 2010), Gravity and Magnetic Methods for Geological Studies (Mishra, 2011), and Field Geo- physics(MilsonandEriksen, 2011).

Organization of this book

This book consists of 14 chapters organized into two principal sections dealing with gravity and magnetic exploration plus an introductory chapter and a final chapter dealing with applications. The initial chapter describes the basis and foundations of the gravity and magnetic methods, the general components of the geophysical process, and the nature of geophysical data. The final chapter considers the application of gravity and magnetic methods to near-surface studies including engineering, environmental, and archaeological investigations, energy and mineral resource exploration, and geologic studies of the lithosphere. Each of the two principal sections consists of six chapters involving an introduction to the method, germane potential theory, the physical property involved in the method, and three final chapters which describe the acquisition, processing, and interpretation of the data. Finally, an appendix deals with data systems processing principles which are important to the processing and interpretation of gravity and magnetic methods. Included in appendix are a discussion of gravity and magnetic data bases and stan- dards, mathematical methods widely employed in gravity and magnetic methods, anomaly analysis, and data graph- ics. It serves as important background to both the gravity and magnetic chapters.

The six chapters of each of the sections are provided on the website that describe the applications of both gravity and magnetic methods. They may be used independently in separate courses or study programs. Overlap between the sections is kept to a minimum. However, descriptions of some methods and principles are more fully developed in the discussions of a particular section. For example, some depth determination methods used in both gravity and magnetic studies are more comprehensively described in the magnetics chapters because they are more widely used and successful in this application. The text explains the need for the reader to study the fuller explanation elsewhere to achieve a comprehensive understanding of the method.

As a study aid to the reader, each chapter is introduced with an Overview and concluded with a summary section listing the key concepts of the chapter. The overview is not an abstract, but rather provides the reader with a broad, generalized summary of the chapter that is a useful guide in reading and studying the chapter. The summary key concepts draw the reader’s attention to the more important concepts that are presented. They are not a listing of what should be known upon reading the chapter, but rather guide the reader in reviewing what has been emphasized in the chapter.

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Preface xi Important elements of the book are the illustrations and

tables which aid in understanding principles, and provide useful data tabulations, and examples of gravity and magnetic methods and their analysis. They are derived both from the literature and from original composition for the book. Numerous references guide the reader to the source of specialized information and additional details on a topic.

Readers seeking additional deﬁnitions of the geoscience terms in geophysics and geology used in this book should consult theEncyclopedic Dictionary of Exploration Geo- physics(Sheriff, 2002) and the Glossary of Geology (Neuendorfet al., 2005), respectively.

A website, www.cambridge.org/gravmag, accompanying the book provides useful supplemental material for the user of the book. Several black and white illustrations in the book are reproduced in color on the website to enhance their utility to the reader. Additionally, a few complex ﬁgures are shown in expanded size for improved leg- ibility. Four additional chapters are provided on the website that describe the applications of both the gravity and magnetic methods to near-surface investigations, energy resources, mineral resources, and lithospheric investigations. The application chapters review the use of the methods and cite examples. The examples demonstrate the scale and magnitude of the anomalies, their breadth of application, and the limitations of the methods. The selection of examples is necessarily limited, but we have attempted to reach a balance in the selection process of including traditional applications plus unique examples to illustrate the range of applications. Placing these chapters on the website has permitted greater coverage, extensive use of color in the illustrations, and provision for updating examples with improvements in technology and broadening the range of problems addressed by the gravity and magnetic methods.

Study questions and exercises

In addition to the supplemental material, the website con- tains study questions for the ﬁrst 13 chapters of the book.

These questions foster further understanding of the topics of these chapters and serve as a study guide for the resder and a resource for instructors. Exercises problem sets and practical examples of application of the gravity and magnetic methods are also presented on the webpage

www.geosoft.com/ website. Exercises have been developed with the cooperation of Geosoft, Inc. utilizing a special version of their software, Oasis montaj. The reader of this book can readily access the software through the link provided on the website. The Geosoft software. The software should be used to process and interpret the data supplied for the exercises. The exercises are keyed to spe- ciﬁc sections of chapters of the book where the user can gain knowledge of the basis, background, and application of the methodologies employed in the programs. Effort will be made to keep the software available as well as the exercises up to date with current practice in the gravity and magnetic exploration profession. Updates of the content are identiﬁed on the webpage.

Some of the exercises on the website involve forward calculation of anomalies, both gravity and magnetic, and discussion of the results of the modeling. These calculations can be performed using the special Oasis montaj version for this book. Alternatively, the software for modeling, inversion, and ﬁltering of proﬁle gravity and magnetic anomaly data prepared by Prof. Gordon R. J. Cooper, University of Witwatersrand can be used.

The latter software can be obtained on the website http://www.wits.ac.za/academic/science/geosciences/

research/geophysics/gordoncooper/6511/software.html.

Descriptions of these programs and their use are presented with the software.

Units

The units used in geophysical methods are diverse, depending largely on the application of the methods. In this book the normal practice is to use SI units. SI is the abbreviation for Le System International d’Unites, which is a system of units that is broadly accepted internationally by governmental agencies and professional societies. This system has similarities to the metric system of units, but is not identical to it. The base and supplementary units of the SI system together with their combinations, called derived units, that are common to geophysical studies in this book are listed in Table 1.2 of Chapter 1. In this book the exception to the use of SI units is in the description of certain case histories. The original units used in reporting the results of the study, which may not be SI units, are used for consistency with the original information.

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Acknowledgements

We wish to acknowledge our colleagues and students who have encouraged us to write a modern book on the gravity and magnetic methods and supported us in this effort. We acknowledge the contribution of the follow- ing colleagues who have reviewed early drafts of one or more sections of the book: Mohammad F. Asgharzadeh, Lawrence W. Braile, Val W. Chandler, David A. Chapin, John D. Corbett, Hyung Rae Kim, Dmitry Koryakin, Xiong Li, Neil M. Coleman, Dhananjay Ravat, Michal Ruder, Richard W. Saltus, Patrick T. Taylor, and Daniel Winester. Their advice has been invaluable to us, but they are not responsible for errors of omission or com- mission in the ﬁnal manuscript. We also acknowledge the cooperation of Colin Reeves in making available his useful web-based publication entitled “Aeromagnetic Surveys.”

Special thanks are extended to Ian MacLeod, Elizabeth Baranyi, and Gerry Connard of Geosoft, Inc. for their continuing cooperation in providing software and assistance in organizing and implementing geophysical processing and interpretation exercises for this book. We are particularly grateful to Elizabeth Baranyi of Geosoft for prepar-

ing the geological model and its anomaly ﬁelds shown on the cover of this book and assisting the authors in the preparation of the exercises on the website that employ Geosoft software. We also thank Kevin Ahlgren of Ohio Stste’s School of Earth Sciences for assistance with designing and implementing the online exerises.We also thank Kevin Algren of Ohio State’s School of Earth Sciences for assistance with designing and implementing the online exercises. We acknowledge with gratitude the cooperation of Prof. Gordon R. J. Cooper of the University of Witwatersrand, Johannesburg, South Africa in providing the users of this book with ready access to his computational software. Yuriy Yeremenko has provided invaluable assistance in preparing the ﬁgures. For this we express our deep appreciation. We also thank Kevin Ahlgren of Ohio Stste’s School of Earth Sciences for assistance with designing and implementing the online exercises.

We also want to acknowledge the continuing assistance from our Cambridge University Press editor, Laura Clark, and also her patience in dealing with three authors with a wide variety of commitments that complicated completion of the book.

xii

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List of symbols

∀ for all or for any

∈ in

not in

≡ if and only if

→ implies

such that

⊃

xiii

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Part I

Introduction

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1 Introduction

1.1 Overview

Geophysics, the science of the physics of the Earth from its magnetosphere to the deep interior, is useful in characterizing the subsurface Earth. Solid-Earth geophysics employs techniques involving the measurement of force ﬁelds to study subsurface features and the processes that act upon them. Thus, geophysical studies serve a broad variety of geologic, natural resource, engineering, and environmental purposes. Gravity and magnetic methods, which measure very small spatial and temporal changes in the terrestrial gravity and magnetic force ﬁelds, have a wide range of uses from submeter to global scales.

Although these methods in most cases fail to match the resolution and precision of direct observations, they are rapid, cost-effective, and non-invasive procedures of studying the inaccessible Earth and optimizing the location of drill holes for direct studies and other remote sensing studies which have higher resolution capabilities.

The application of gravity and magnetic methods generally involves a common approach consisting of planning, data acquisition, data processing, interpretation, and reporting phases. During the planning phase the appropriate method(s) are selected for meeting the objective of the study, and procedures for data acquisition, processing, and interpretation are established. These decisions are reached on the basis of experience, model studies, or test surveys. Special care is taken to determine an error or noise budget for the survey and to consider the propagation of errors, both random and systematic, through the data acquisition and processing chain. Selection of the distribution of observations in the survey region includes consideration of the objective of the study, the geologic, topographic, vegetative cover, and cultural features of the area, access over the region, and ﬁnancial considerations. The geophysical observations are subject to numerous analytical processing steps to minimize effects from non-germane sources. Interpretation of these processed data involves not only determining the distribution of anomalous masses in the subsurface, but the nature of these masses. The latter commonly requires the translation of properties directly measured by the geophysical method into secondary properties, such as lithology, porosity, and strength, which are more immediately related to the survey objective. Interpretation is achieved by transforming the survey data to quantitative models of the subsurface that satisfy the data. However, all interpretations are subject to ambiguities that to a degree depend on the implemented method and procedures and the integration of the results with collateral geological and geophysical information.

1.2 The Earth and its planetary force ﬁelds Geophysics is an interdisciplinary science that integrates the observations, hypotheses, and laws of geology with the techniques and principles of physics to understand the

composition, nature, structure, and processes of the Earth.

Geophysics involves measuring and interpreting phenom- ena related to the physical nature of the Earth, from its center some 6,371 km beneath the surface to the outer limits of its magnetosphere at altitudes many times the 3

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4 Introduction

Mohorovicic discontinuity

Solid Gutenberg

discontinuity ^{2,885 km}

COMPOSITIONAL LAYERING RHEOLOGICAL LAYERING

6,371 km 650 km

370 km

−

~700 km m

z

m

5,144 km Inner core

FIGURE 1.1 Cross-section of a segment of the Earth showing major ﬁrst-order internal subdivisions in composition and mechanical or rheological properties. Table 1.1 lists the mean densities and magnetizations of the Earth’s major structural components. Adapted from K e a r yandV i n e(1990).

radius of the Earth. Thus, it incorporates investigations of the subsurface, hydrosphere, atmosphere, ionosphere, and magnetosphere. In this book, the focus is on solid-Earth geophysics, considering the properties and processes of the Earth primarily within the crust and uppermost mantle (lithosphere) as reflected in the spatial and temporal variations in gravity and magnetic force fields. We are all very aware of these planetary fields. The gravity field is the source of the force which causes all objects to be attracted toward the Earth, and the geomagnetic field controls the compass which is useful in determining geographic direc- tions. These force fields have been and continue to be an important part of the science of geophysics.

Applications of gravity and magnetic methods include micro-scale surveys to map the physical property variations of the upper meter or two of the subsurface, or conducted within drill holes to establish the physical properties of the adjacent rocks. Larger-scale applications include regional to global surveys designed to image the deeper variations of the Earth’s crust, mantle, and core (Figure 1.1). The crust is the outermost surface rind that consists of surface-like rocks extending to depths as great as 70 km. The crust overlies the mantle, made up of higher density and velocity but generally non-magnetic rocks and

TABLE 1.1 The average densities< σ >and magnetizations

< J >in kg/m³and A/m, respectively, of the Earth’s major structural elements shown in Figure 1.1.

Structure < σ > < J >

Upper crust 2,200–2,900 0–5

Lower crust 2,800–3,100 2–10

Upper mantle 3,300 0

Asthenosphere 3,300–4,000 0

Lower mantle 4.400–5,500 0

Outer core 9,900–12,200 8.4×10¹⁵

Inner core 12,800–13,100 0

extending to a depth of roughly 2,900 km, which in turn lies directly on the roughly spherical, dense, largely metallic core of the Earth in which the main terrestrial magnetic ﬁeld originates. The lithosphere is the outermost semi- rigid shell consisting of the crust and uppermost mantle.

It normally has a thickness of roughly 150 km beneath the continents and less in oceanic regions, and is the source of most of the variations in the gravity and magnetic ﬁelds of the Earth.

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1.3 Basis of the gravity andmagnetic methods 5 The crust exhibits highly complex structural and com-

positional properties that reflect the effects of erosion, sed- imentation, metamorphism, tectonics, and igneous activ- ity, and the plastic movement of the mobile asthenosphere underlying the lithosphere that have occurred over the Earth’s 4,600-Myr history. These processes have led to the differentiation of chemical elements, deposition of a variety of sediments, vertical and horizontal movements, zones of crustal weakness, and the focusing of geological processes, such as volcanism, in limited regions of the Earth. These variations in the nature of the lithosphere and specifically the crust that solid-Earth geophysicists map and investigate are of societal interest because they con- trol the formation and distribution of the Earth’s resources, and volcanic, earthquake and other natural hazards. Geo- physics is an efficient and effective method of conducting these investigations, avoiding the problems of direct sampling of the hidden Earth. Nonetheless, these studies come at a cost, because the results of their interpretation are to varying degrees ambiguous and lack the accuracy of direct measurements.

1.3 Basis of the gravity and magnetic methods

Gravity and magnetic methods are commonly referred to as potential field methods because the measurements involve a function of the potential of the observed field of force, either the terrestrial gravity or magnetic field, at the observation site. These methods are widely used at a variety of scales to investigate the Earth because in com- parison to most other geophysical methods the acquisition of data is inexpensive and rapid, and for many applications the reduction and interpretation of the observations are relatively simple. Furthermore, gravity and magnetic methods always provide information about the subsurface.

In addition, there is a large reservoir of these data cover- ing the entire Earth in varying detail that are publically available at minimal cost to the user.

1.3.1 Gravity

The gravity method involves measurement of very small variations in the Earth’s gravitational ﬁeld, of the order of a few parts per million or lower, caused by lateral variations in density. Most observations are made with highly specialized weighing devices, called gravimeters or gravity meters, which measure the acceleration of gravity. Less frequently, they are made with instrumentation which mea- sures the gradient or vector components of the gravity ﬁeld.

Gravity variations useful for studying the solid Earth are observed on land, in surface and subsurface water ves- sels, in drill holes, in the air, and from satellites orbiting the Earth. The variations they measure are dependent on Newton’s universal law of gravitation, which takes into account the differential mass and the distance between the source and observation point. Because density is a universal property of matter, gravity is ever-present, but only where the density of the Earth varies laterally will gravity variations be noted that can be related to changes in the nature and structure of the Earth.

Observed gravity variations called anomalies are the differences between the observed and the theoretical ﬁeld based on planetary considerations and the assumption of radial symmetry of the Earth layers. The anomalies may be either positive or negative depending on the presence of mass excesses or deﬁciencies, as illustrated in Figure 1.2. Their interpretation is subject to uncertainties in the observation, reduction to anomaly form, and processing and limitations resulting from the inherent ambi- guity in their interpretation. However, meaningful interpretations can be obtained with proper use of constraining, collateral geologic and geophysical information.

A wide range of densities occurs within the crust, from the essentially zero density of air-ﬁlled voids in near-surface formations, to densities of unconsolidated sediments with their interstitial openings ﬁlled with either air or water, to the highest densities related to iron/magnesium-rich crystalline rocks and metallic ores.

Even higher densities are associated with the radial shells that make up the mantle and the core of the Earth. The potentially broad range of contrasting densities in the near- surface, in the crust, and in subcrustal rock materials leads to the wide range of applications of the gravity method.

1.3.2 Magnetics

The magnetic method (commonly referred to as magnetics) is similar to the gravity method in that variations in a planetary field are measured, in this case the magnetic field of the Earth. Observations are readily made to a high precision with portable electronic magnetometers on land, drill holes, sea, and air including measurements from planet- orbiting satellites. Most land areas of the Earth have now been measured by airborne observations and much of the ocean area has been observed either by airborne or ship- borne measurements, at least by widely spaced observations. Variations, or anomalies, in the magnetic field of the Earth obey Coulomb’s law, which is comparable to Newton’s law of gravitation, but takes into account the magnetic polarization variations of the Earth rather than

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6 Introduction

(A)

(B)

(A) (B)

(A) + (B)

Δσ = 100 kg/m³

Δσ

Δσ = 100 kg/m 3

Δσ

Δσ = 100 kg/m³

(a)

(b)

(c)

Distance (km)

Depth (km)Depth (km)Depth (km)Anomaly (mGal)Anomaly (mGal)Anomaly (mGal)

30.0

5.0 10.0 15.0 20.0

5.0 10.0 15.0 20.0 30.00 25.00 20.00 15.00 25.0 20.0 15.0 10.0 5.0

25.0 20.0 15.0 10.0 5.0

100 90

40 50 60 70 80

30 20 10

100 90

40 50 60 70 80

30 20 10

100 90

40 50 60 70 80

30 20 10

FIGURE 1.2 Examples of vertical gravity effects derived from faulting that offsets a layer of higher density within a formation of lower density. Panel (a) shows the layers vertically offset with the proﬁles of the gravity effects for the individual layer components (A) and (B), as well as for their total or superimposed effects (A+B). Panels (b) and (c) show the total gravity effect proﬁles for normal and reverse faulting of the horizontal layers. The illustrated gravity effect is the vertical acceleration of gravity given in units of milligals (mGal) where 1 mGalis equal to10⁻³cm/s²or 10⁻⁵m/s².

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1.4 Foundations of geophysical methods 7

Anomalous field

Earth’s field Pole Equator

+

_ Anomaly (nT or )0

Observation Surface

Earth’s field Source field Total field

-

+

FIGURE 1.3 Example of the total magnetic field variation (anomaly) caused by the magnetic effect of a buried magnetic object (shaded area) magnetized by the Earth’s field. The sum of the magnetic field of the anomalous source and the Earth’s field produce the observed total magnetic intensity anomaly profile at the top of the figure. Note the asymmetry of the anomaly owing to the inclined magnetization and the negative component of the anomalous field caused by the positive pole of the magnetic source.

its mass. Magnetic polarization is dependent on the magnetic susceptibility and the remanent or permanent magnetization of Earth materials. Magnetic susceptibility is a measure of the ease with which a material can be magnetized in the current magnetic ﬁeld of the Earth; remanent magnetization is the permanent magnetization previously acquired and retained by the material.

All magnetic materials, including the Earth, have two poles, north and south or positive and negative, and thus are called dipolar. Objects of high magnetic susceptibility become polarized or magnetized when they are present in the Earth’s dipolar geomagnetic field. Magnetic field observations taken over a buried magnetized object will measure both the positive and negative fields associated with the dipolar magnetization of the object. The resulting anomaly from a high magnetic susceptibility object will combine the fields of both poles as, for example, illustrated in Figure 1.3, but will be dominated by the pole nearest to the observation. The magnetic field of the Earth will induce in the northern geomagnetic hemisphere a negative pole near the top of the anomalous source and a positive pole near the base of the source. The negative pole, being in closer proximity to the observations, will produce a greater attraction on a north-seeking pole (+) than the

repulsion from the nagative pole. Accordingly, the magnetic field over the anomalous source will be dominated by an increase in the magnetic field over the Earth’s field as shown in Figure 1.3. An inverse anomaly dominated by a decrease in the field would be observed over an object with lower magnetization than the surrounding Earth materials. The magnetization of an object magnetized in the Earth’s magnetic field will align with the Earth’s field. As a result the anomalous field will vary with location on the Earth’s surface owing to the dipolar nature of the main field which roughly aligns with the axis of rotation and the resulting changes in the main field over the Earth’s surface.

Unlike crustal rock densities, which generally vary by less than a half-order of magnitude and are directionally independent, magnetic polarization commonly varies over several orders of magnitude, giving rise to large property contrasts, and is directionally variable. The direc- tional attribute of magnetization complicates the interpretation of measurements of the magnetic ﬁeld, as do the presence of both positive (attractive) and negative (repul- sive) poles within all magnetic materials as illustrated in Figure 1.3.

An advantage of the magnetic method over the gravity method is that the ﬁeld varies inversely one power faster with distance to the source than does the gravity ﬁeld from the same source. As a result, the magnetic method is more sensitive to the source depth, which is commonly an important objective in interpretation of the observations.

Furthermore, the resolving power of the method to distin- guish independent sources is greater than that of the gravity method. Magnetic ﬁeld variations are derived from only a few minerals, and these occur only as accessory minerals in most rocks. However, the measured variations are several parts in a hundred thousand or greater; thus magnetic variations are easier and less costly to map than are gravity anomalies from similar sources. Also, magnetic measurements can readily be made from simple, mobile platforms increasing the surveying rate, making them cost-efﬁcient.

As a result the magnetic method is widely applied as a reconnaissance tool in geophysical studies and has several specialized applications in shallow subsurface and crustal studies.

1.4 Foundations of geophysical methods The foundations of geophysics were developed in the last few centuries through scientiﬁc studies of surface geological features by pioneering geologists and the study by early physicists of natural force ﬁelds of the Earth. From the seventeenth century onwards, geologists such as Steno,

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8 Introduction

Smith, Werner, Hutton, Playfair, and Lyell established the basic laws of geology, and explained the formation of rocks. These and other principles are explained in introductory geological texts. The reader unfamiliar with the basics of geology is encouraged to learn the key concepts presented in these books because they are fundamental to the understanding and application of geophysical methods in general, and the gravity and magnetic methods in particular.

Contemporaneous to the early geologic studies, physicists investigated a variety of terrestrial force ﬁelds and developed theories and laws to explain their observations.

Beginning in the sixteenth century, such prominent scientists in the history of physics as Newton, Galilei, Gilbert, Gauss, Coulomb, Volta, Oersted, Ampere, Bouguer, Fara- day, Fresnel, and Maxwell contributed greatly to the science of geophysics. By the mid-nineteenth century, they and their peers had essentially established the foundations of gravitational, magnetic, and electrical fields of the Earth and the basic theory upon which are based current studies of these fields in geophysics. At about that time, instrumentation was becoming available for field geophysical measurements and the potential for subsurface studies with these measurements was being identified.

Building upon improved instrumentation and interpretational techniques developed in the succeeding decades of the twentieth century, particularly after World War I, great progress was made in the use of gravity and magnetic methods in the search for Earth resources. Technologi- cal developments, primarily in electronics, during World War II made instrumentation improvements that led to the broad use of computers, electronic magnetometers, accelerometers, ground-penetrating radar, digital record- ing, and other advanced instrumentation of geophysics.

Post-World War II geophysical investigations in geomagnetism, seismology, paleomagnetism, and isotopic age dat- ing of rocks led to the development of the paradigms of seaﬂoor spreading and plate tectonics by Vine, Matthews, Morley, Morgan, Sykes, Runcorn, Oliver, Wilson, Heirt- zler, Dalrymple, and others. These paradigms explain the slow movement of crustal units over the Earth’s surface as well as the destruction of existing crust and the construc- tion of new crust by interaction with the Earth’s mantle.

These concepts are essential to understanding the evo- lution and the geological and physical processes of the Earth, and thus to the application of gravity and magnetic methods.

The latter half of the twentieth century saw technological improvements that resulted in more precise, portable, and inexpensive instrumentation and faster computations.

The continuing improvement in computers has been fun-

damental to all of geophysics. Not only have computers made it possible to collect and store huge amounts of data, but they are the keystones to current data processing and presentation technology responsible for today’s broad suc- cess and acceptance of geophysics.

Progress in geophysics has been driven by societal needs and economic factors as well as by technological advances. In the 1920s and 1930s, the worldwide surge in the number and use of automobiles, with their gasoline- powered internal combustion engines, increased the need for petroleum products. This need could not be met solely with production from petroleum traps located by surface geologic information and wildcat drilling. Geophysics stepped into this void by greatly increasing the chances of discovery. The growth in petroleum exploration geophysics was accelerated by the ever-increasing demands of the post-war surge in the world’s economy. The societal and economic pressure caused a revolution in petroleum exploration geophysics that continues today. In a similar way, post-World War II industrial developments and the depletion of mineral resources during the global war forced the broadening of mineral exploration to the geophysical search for new mineral districts and ore deposits which have little or no surface indication.

Petroleum geophysical exploration began with instru- mental developments in the early part of the twentieth century that permitted gravity to be measured with a precision necessary to study subsurface geologic structures.

These developments led to the first geophysical discovery of petroleum in the United States, which was by the gravity method, in the early 1920s. The use of gravity in petroleum exploration reached a peak shortly after World War II, but its relative role decreased as the reflection seismic method was improved, largely as a result of the computational power of computers and related theoretical developments. Nonetheless, the gravity method has a significant niche and is especially valuable used in concert with the reflection seismic method to constrain possible interpretations. The improvements in the gravity method for hydrocarbon exploration have given impetus to its use not only for this application, but also for shallow zone and regional exploration of the Earth. The successful development of techniques for measuring gravity to a precision useful for exploration using airborne and satellite platforms has given the method a new range of applications.

The magnetic method has been used since the seventeenth century in mineral exploration, especially for iron ore prospecting, but with the advent of airborne magnetic observations after World War II, it has been used on a broad basis for regional geological studies in petroleum

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1.5 Geophysical practices 9

FIGURE 1.4 The geophysical practice for implementing the gravity and magnetic methods involves a sequence of six phases.

exploration, both detailed and regional studies in mineral exploration, and in geologic mapping of crystalline rock terrains where the rock units have varying magnetic polarization. It has also proved useful in identifying ferrometallic objects in the near surface, such as buried well casings, storage containers, and unexploded ordnance and the study of archaeological sites.

The development of satellites and other space age tech- nologies since the 1960s has greatly advanced regional exploration of the surfaces and deep interiors of the Earth and other planetary bodies. Political considerations do not limit satellite operations so that essentially any region is available to satellite remote sensing and geophysical mapping efforts. Unprecedented timing and positioning data from the constellation of Global Positioning Satellites (GPS) greatly expedite modern geophysical survey efforts, while the communication capabilities of satellites allow access to geophysical experiments literally worlds away from our ofﬁces and laboratories. Satellite gravity and magnetic observations, in particular, are yielding important new insights on the nature, architecture, and dynamics of the Earth and other terrestrial planets.

1.5 Geophysical practices

The gravity and magnetic methods are described in individual chapters that follow. However, in addition to the fundamentals specific to the individual methods, there are general principles and practices that are used in geophysical exploration programs. They are sufficiently general that a description of them serves as an introduction to the use of both gravity and magnetic methods. Whether they deal with the selection of the geophysical method, the design of a data acquisition and processing program, the reporting of an investigation or any one of the numerous components that mark a successful geophysical campaign, they are for the most part nothing more than the application of appropriate scientific methodology (Figure 1.4). The fol- lowing description assumes that the program involves all phases from planning to report preparation and archiving the data and results. Programs may also focus on previously surveyed data that already have been reduced to anomaly form, where only the latter phases are applica- ble. Nonetheless, considering the factors for the phases described below will help to determine the usefulness

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10 Introduction

of existing data. In the individual sections dealing with the gravity and magnetic methods, descriptions are provided of the data acquisition, processing, and interpretation phases, but the planning phase is focused on the principles of the methods and the survey objectives.

1.5.1 Planning phase

The planning phase is perhaps the most important step in the geophysical approach because it is in this stage that fundamental decisions are made regarding the nature and procedures of the program. Appropriate planning requires collecting and using all available geological and geophysical data and interpretations, and establishing strong communication links among the interested parties regardless of their particular expertise. Plans should only be ﬁnalized after all parties have had the opportunity to interact.

Planning is subdivided into two segments: ﬁrst, the selection of the appropriate method(s) and, second, the design of the survey and the subsequent data processing and interpretation. To be successful, both require a clear exposition of the objective of the survey. Important collateral information is the speciﬁcation of the volume of interest to the survey – that is, the areal as well as the depth extent of interest. This subsurface volume is limited as much as possible within the framework of the problem because the areal extent of the survey is a major factor in determining the cost of the survey. In addition, the survey procedures are tuned to the depth of interest as dictated by the survey objectives.

The most important consideration in the selection of the method for a study is to determine if the target sources will produce an observable anomaly even in the presence of extraneous signals. This requires estimation of the anticipated source volume, depth, and physical property contrast as well as evaluation of potential geologic, observational, and processing noise and errors. Informa- tion on the physical properties of the Earth materials in the subsurface volume being investigated is important to all phases of the application of geophysical methods, but particularly in planning studies when target anomalies are being estimated. Rock property data are obtained from in situmeasurements on the site, sample measurements, and general tabulations. The character of target anomalies may come from experience in related situations, forward modeling of both anticipated anomalies and potential anomaly noise, or test surveys. The latter are particularly useful where information needed for modeling of sources and estimating noise and errors is lacking. The parameters of the source targets commonly cover a range of values necessitating the study of a distribution of anomaly

characteristics. Evaluation of anticipated anomalies in reference to the objectives of the investigation may sug- gest the use of multiple methods. The combined use of gravity and magnetic methods is particularly powerful in studying crystalline rock terrains that consist of large volume sources with both density and magnetic polarization contrasts.

Once the optimum geophysical method or methods have been selected for a study, the survey must be designed to accomplish the objectives in a minimum time at the low- est possible cost without jeopardizing the quality of the survey. The anticipated signal from the anomalous geologic features of interest will dictate many of the attributes of the survey design. Survey design is a matter of maximizing the information obtained and required within the ﬁnancial limits of the survey. This is often accomplished with a heuristic method based on experience and knowledge of ﬁeld characteristics, or on a statistically based experiment design methodology as described byCurtis(2004a and 2004b). The areal coverage of the survey, of course, will be a function of the size of the study area and the anticipated size, depth, and depth extent of the anomalous features.

The greater are these parameters, the larger the required size of the survey area. The anomalies often must be isolated from regional and noise effects, thus the survey area must extend well beyond the study area or the areal con- ﬁguration of the anomalous feature. This is well illustrated for both the gravity and magnetic methods in Figures 1.2 and 1.3, respectively, where the anomaly needed for iden- tiﬁcation and analysis of the subsurface feature (that is, the fault in the gravity anomaly illustration and the ferrous source in the magnetic anomaly illustration) extends well beyond the immediate region of the anomalous feature.

Critical concerns in planning surveys are selection of the data density and precision. These are determined by the objectives of the survey and characteristics of the anticipated signals. For most objectives the anomalous signal including the maximum gradients must be fully measured, not simply the maximum amplitude of the anomalous signal. In gravity and magnetic surveys it is necessary to map the gradients of the anomalies to effect a useful interpretation. This requirement necessitates closely spaced and high-precision observations. Forward modeling of the range of anticipated anomalies, including their size, properties, and position, provides a basis for selecting the required data density and precision.

In general, sampling theory speciﬁes that the station interval should be no greater than half the length of the smallest dimension that needs to be mapped in the survey. This interval or spacing is referred to as the Nyquist wavelength and its inverse as the Nyquist frequency or

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1.5 Geophysical practices 11 wavenumber in the spatial domain. It is the maximum

spacing that should be used between measurement points, although in practice the sampling interval should be considerably less than the Nyquist sample interval if the gradients of the measurements are of interest or the higher- frequency noise components must be mapped and isolated from the desired signal. In gravity and magnetic methods, the separation between observations is directly propor- tional to the depth of investigation – that is, the greater the target depth, the greater is the permissible station spacing.

Often a separation approximately equal to half of the depth to anticipated sources is used in surveys. However, gener- alizations regarding separation of measurement points are of limited value because of the need to consider the spe- ciﬁc attributes of the survey. As a result, it may be desirable to determine quantitatively the probability that a speciﬁc anomaly will be detected utilizing the sampling theorem, with stations located either randomly through a region or on a regular grid (e.g.SambuelliandStrobba, 2002).

Wherever possible, it is desirable to conduct test surveys over a limited, representative portion of the survey area or noise tests to select the optimum survey layouts.

1.5.2 Data acquisition phase

In the data acquisition phase, the actual field and necessary related data are measured and recorded. Auxiliary observations that are made in addition to the primary geophysical measurements include essential data for the reduction of the measurements to an interpretable form (e.g. station elevation or flight altitude, surrounding topographic relief, and water depth). Instrumentation must be selected to meet the precision requirements of the survey as established in the planning phase as efficiently as possible. Actual field procedures are dictated by the survey objectives, sources of noise, surface and weather conditions, instrumentation, and access within the survey area. For example, access may limit the survey to discrete observations along roads rather than a grid pattern more useful in interpretation. Observations are commonly made along traverses which are oriented perpendicular to the prevailing strike direction of the anomalies, separated at greater distances than the observations along the traverse. The distance between traverses is determined by estimating the length of the continuity of the character of the anomalies along their strike direction.

Gravity and magnetic methods are particularly effective in a reconnaissance or regional study mode because they are fast and efﬁcient; while other methods may provide better detail and resolution, they are likely to be more costly and time-consuming. Use of gravity and magnetic

methods early in an exploration program sequence can delimit an area for detailed investigations with other methods and improve the survey design to obtain maximum information from measurements. For example, magnetic measurements which often can be taken quickly and inex- pensively from an airborne platform may be used to delin- eate likely faulted areas. In this way limited sectors of a large region can be isolated for study and evaluation in much greater detail by slower and more costly methods, like the seismic reﬂection method. Similarly, regional gravity surveys may be used to determine the strike direction of prevailing geological features within an area which can guide the selection of the direction of detailed traverses along which gravity or other geophysical ﬁelds or forces are measured.

1.5.3 Data processing phase

The nature and role of the data processing phase may vary considerably between gravity and magnetic methods and with the survey objectives. In general, the data processing requirements of the magnetic method are considerably simpler than for the gravity method largely because of the intense magnetic polarization contrasts in the crust of the Earth. For example, in some magnetic studies to locate near-surface ferrometallic bodies, the amplitudes of anomalies are sufficiently large that no data processing is needed. However, this is the uncommon situation, particularly with the increasing demands for precision in the results of geophysical studies. Accordingly, most survey objectives and methods lead to data in which the signal to be used in interpretation is significantly dis- torted by extraneous effects. Data processing is used to remove these extraneous effects and enhance the desired signal for interpretational purposes. Generally, data processing is performed subsequent to acquisition of the raw or observed data, but field processing may be used to minimize unwanted signals. For example, the stabilization of a gravimeter in field procedures may minimize wind-driven accelerations acting on the meter, and field processing by digital filtering can be used to supplement the effect of the field procedure.

Data processing may include several steps. The ﬁrst is to prepare the data for interpretation by removing the effects of instrumentation as calibration adjustments and correction for instrument instability. These data are then reduced for known or predictable eff

Gravity and Magnetic Exploration- Principles, Practices, and Applications. Willam J. Hinze, Ralph R. B. Von Frese, Afif H. Saad. 2008

Gravity and Magnetic

Contents

Preface

Objectives of this book

Related books

Study questions and exercises

Units

Acknowledgements

List of symbols

1.1 Overview

COMPOSITIONAL LAYERING RHEOLOGICAL LAYERING

1.3 Basis of the gravity and magnetic methods

1.5 Geophysical practices